Field of the Invention
The present invention relates to an image processing apparatus, which is configured to process a medical image, an image processing method, and a program.
Description of the Related Art
In recent years, image quality of medical images taken by three-dimensional imaging device, such as an X-ray computed tomography apparatus (X-ray CT) and a magnetic resonance imaging device (MRI), has dramatically improved, and detailed information on the inside of a human body can now be obtained. However, the number of images has increased with the improvement in image quality, and a burden of reading the images on radiologists has increased. As a result, there is an increasing expectation for computer aided diagnosis (CAD).
It is particularly important to automatically detect an abnormal shadow region, which is highly likely to be an abnormal tissue, such as a tumor, or automatically distinguish benignancy and malignancy of an anatomical tissue. For example, in processing for automatically detecting an abnormal shadow region and processing for distinguishing benignancy and malignancy of a tissue by CAD, there is a need to calculate a shape feature amount representing a shape of a contour (boundary) of a site of a living body, which is an object, to thereby automatically calculate a likelihood of an abnormal shadow or automatically calculate malignancy of the tissue.
In the processing for distinguishing benignancy and malignancy of a pulmonary nodule, there have been reported that a lobulated contour (boundary) of the pulmonary nodule is one indicator of malignancy, and that a polygonal or polyhedral contour of the pulmonary nodule is one indicator of benignancy.
However, such an abnormal tissue is often included as a shadow (hereinafter referred to as “attached abnormal shadow”) of a tissue (or site) in a state of being adhere to an organ or blood vessels in a medical image. Further, a surface (hereinafter referred to as “attached surface”) that is adhere to the organ or blood vessels and a surface (hereinafter referred to as “non-attached surface”) that is not adhere to the organ or blood vessels often have different shape feature amounts. A shadow of a tissue (or site) in a state of being not adhere to an organ or blood vessels is referred to as a “non-attached abnormal shadow”.
Clinically, a medical doctor observes a contour (boundary) of a non-attached surface of an attached abnormal shadow to diagnose a tissue. Therefore, in calculating a shape feature amount of a region of the attached abnormal shadow by CAD, when the shape feature amount is calculated including a contour (boundary) of an attached surface, the calculated shape feature amount may differ from a shape feature amount of the contour (boundary) of the non-attached surface that the doctor wants to observe.
To address the above-mentioned problem, in Japanese Patent Application Laid-Open No. 2009-225990, there is disclosed a technology for calculating a circularity using a non-attached surface. Specifically, there is disclosed a technology of obtaining a plurality of nodes forming a polygonal line that approximates a contour of a nodular region, determining a position of a reference point, and calculating a circularity using areas of a plurality of regions determined based on the plurality of nodes and the reference point.